A number of techniques have been previously proposed for obtaining images of a formation at the wall of a borehole. Such images are useful in understanding the geology and lithology of the formation. Such information can be particularly useful when drilling boreholes such as oil and gas wells since it can help to identify the presence of desirable fluids and provide information allowing effective completion of the well.
The use of video technology for inspecting the inside of boreholes is known. An example can be found in U.S. Pat. No. 5,790,185. This technique is typically used to obtain images of large scale structure in the borehole, for example the state of casing or perforations. Such an approach requires the presence of a transparent fluid in the well and so cannot be used in the drilling or production phase of a well in which opaque fluids are present.
Certain high-resolution imaging techniques have been proposed for use in the drilling and production phase of the well. Such techniques include measuring properties of the formation such as electrical properties (local conductivity or permittivity), acoustic properties (acoustic impedance or formation slowness) or nuclear properties (density, photoelectric effect). These high resolution measurements made at the borehole wall can be displayed as a two-dimensional image. U.S. Pat. No. 4,567,759 discloses one such example of this approach.
U.S. Pat. No. 6,140,637 describes a technique for detecting the presence of hydrocarbon by fluorescence measurements. The tool described comprises a light source and a sensor in a tool body connected to a sensing head by means of an optical fibre bundle. The sensor head includes a window that is pressed up against the borehole wall so that light illuminating the borehole wall can cause hydrocarbons to fluoresce and the resulting light passed back to the sensor for detection. The purpose of this measurement is to detect the presence of hydrocarbon. The nature of the measurement does not allow an image to be obtained.
U.S. Pat. No. 3,091,235 describes a form of diagnostic instrument that has become known as a fibrescope or endoscope. A optical fibre bundles are used to illuminate a remote site and transfer an image back for analysis. A coherent fibre bundle is used to ensure that the image is properly transferred.
This invention aims to provide techniques that allow high-resolution optical images to be obtained of the borehole wall that can allow features of the formation the be viewed that are otherwise unavailable with other imaging techniques.